As a proxy for the strength of the green movement they used the voting records of members of the US Congress from each state, as rated by the League of Conservation Voters, an environmental group. “The voting record is a pretty good indicator of how green the state is,” says Dietz.

They found that the green movement can counteract emissions, which keep growing as a result of population expansion and affluence. For example, a 1 per cent increase in a state’s environmental score more than neutralises typical annual increase in emissions (PNAS, doi.org/5d9). “States with higher levels of environmentalism have lower emissions than we would otherwise expect,” says Dietz. “Also, as states become more environmentalist over time, their emissions increase less than you would otherwise expect.”